1. Field of the Invention
This invention relates to the fabrication of a perpendicular magnetic recording (PMR) write head whose main pole is at least partially surrounded by shields formed of magnetic material. In particular it relates to such a head whose main pole and trailing shield is separated by a write gap formed of antiferromagnetic (AFM) material.
2. Description of the Related Art
The increasing need for high recording area densities (up to 1 Tb/in2) is making the perpendicular magnetic recording head (PMR head) a replacement of choice for the longitudinal magnetic recording head (LMR head).
By means of fringing magnetic fields that extend between two emerging pole pieces, longitudinal recording heads form small magnetic domains within the surface plane of the magnetic medium (hard disk). As recorded area densities are required to increase, these domains must correspondingly decrease in size, eventually permitting destabilizing thermal effects to become stronger than the magnetic interactions that tend to stabilize the domain formations. This occurrence is the so-called superparamagnetic limit. Recording media that accept perpendicular magnetic recording, allow domain structures to be formed within a magnetic layer, perpendicular to the disk surface, while a soft magnetic underlayer (SUL) formed beneath the magnetic layer acts as a stabilizing influence on these perpendicular domain structures. Thus, a magnetic recording head that produces a field capable of forming domains perpendicular to a disk surface, when used in conjunction with such perpendicular recording media, is able to produce a stable recording with a much higher area density than is possible using standard longitudinal recording.
Since their first use, the PMR head has evolved through several generations. Initially, the PMR head was a monopole, but that design was replaced by a shielded head design with a trailing edge shield (TS), which, due to its negative field, provides a high field gradient in the down-track direction to facilitate recording at high linear densities.
Side shields (SS) then began to be used in conjunction with the trailing edge shields, because it was necessary to eliminate the fringing side fields in order to increase writing density still further. Unfortunately, despite the benefits they provided, the presence of these shields inevitably reduces the field produced by the main pole because the basis of their operation is the removal of portions of the flux of that field. Therefore, as long as design functionalities can be achieved, it is important to reduce any additional flux shunting by the shields from the main pole. This is a particularly important consideration for future PMR writer designs which utilize increasingly small pole tips.
In today's quest for very high density magnetic recording it is essential to improve the bit error rate (BER). This requires an increase in the recorded bits per inch (BPI) As the data rate for writing increasing rapidly to the GHz range, it is also important to increase the data rate capability of the writer without losing the BER. At today's state-of-the-art rate of 750 Gb/in2 areal density, the physical width of the writer is reduced to only ≈50 nm (nanometers), with a write gap reduced to sub-30 nm dimensions. The reduction of writer dimensions poses a significant challenge to maintain the write field strength and field gradient for OW, BER and adequate frequency response. This is because most of the writing flux will be shunted from the main pole to the trailing shield without there being an adequate magnetization component along the direction that is vertical to the ABS plane. A critical aspect of writer design, therefore, is to achieve the high writing field and high field gradient by engineering the magnetization configuration and response of the main pole and trailing shield region.
Referring first to schematic FIG. 1, there is shown a side cross-sectional view of components of a prior art PMR write head, with its ABS end (dashed line (60)) positioned over a perpendicular recording type magnetic medium (100) having a magnetically soft underlayer (SUL) (150). There is shown a lead shield (80), a main pole (20), a trailing shield (40), a write gap (65) between the main pole and the trailing shield and a yoke (90). Note that these components generally project backwards (away from the ABS) so that the yoke and main pole have a closed configuration, but that extended view is not shown here. The trailing shield (40) is grown on a high magnetic moment (high Ms) seed layer (45). The medium (100) is moving from the lead shield towards the trailing shield.
During writing, magnetic flux (10) emerges from the main pole (20) and takes two paths. A first path (30) is directly shunted to the trailing shield (40) through the write gap (65), which drives the magnetization of the trailing shield (50) to be parallel to the ABS (60) of the writer. Since the medium is responsive to a vertical field, this flux component is not useful for writing and it should be reduced. Another flux path (35) emerges from the pole tip, passes through the soft magnetic under layer (SUL) (150) at the bottom of the magnetic medium and returns to the trailing shield (40). This component of the flux is the one actually doing the writing on the medium. For good write performance the flux emerging from the main pole and entering the medium needs to have a strong vertical (perpendicular to the ABS) component and it should have some vertical component relative to its re-entrance into the ABS of the trailing shield to efficiently close the flux loop. Therefore, it is advantageous to increase the vertical magnetization of both the main pole and the trailing shield adjacent to the write gap.
The effects of the write field of a prior art configuration such as that shown in FIG. 1 can be obtained from the graph shown in FIG. 2. The graph of FIG. 2 is a micromagnetic modeling result showing the magnitude profile of a down-track write field, as a function of elapsed time after write-current switching. The magnitude, Heff is measured in Oe along the graph ordinate and the down-track position is measured along the abscissa in microns (μm) down track from the pole tip. Five measurement times are superimposed, from 0.5 ns (nanoseconds) to 2.5 ns after the field is shut off.
Two conclusions can be drawn from the graph.
1) the trailing shield magnetization response is lagging behind the main pole field and,
2) the maximum field gradient depends on the positive and negative peak values of Heff and their spacing.
In this modeling experiment, the magnetization of the trailing shield has a component in the same direction as that of the main pole, from times of 0.5 to 1.5 ns, as evidenced by the same polarity of the writing field under the trailing shield. Beginning at 2 ns, however, this trailing shield flux polarity switches direction, providing some anti-parallel component to the main pole magnetization and, thereby, generating a negative dip in the field profile which produces a high field gradient. This effect is greatest at 2 ns and 2.5 ns where the switch in polarity of the field from an Heff of approximately 17 kOe to an Heff of approximately −5 kOe (opposite direction) is due to some component of the trailing shield flux which is anti-parallel to the flux emerging from the pole tip.
These results imply that it will be advantageous to have a writer design which enhances the flux component perpendicular to the ABS between the main pole and the trailing shield that thereby enhances the write field strength and the field gradient. We shall use synthetic antiferromagnetically (SAF) coupled multilayer structures and intrinsically antiferromagnetic materials to achieve the desired design properties. Such structures have appeared in the prior art, but have not been used as in the present invention. Examples can be found in Kief et al. (U.S. Publ. Pat. Appl. 2010/0214692), Van der Heijden et al. (U.S. Pat. No. 6,813,115), Zhang et al. (U.S. Publ. Pat Appl. 2010/0119874, assigned to the present assignee) and Tagami et al. (U.S. Pat. No. 7,443,633).